A typical transmission line type phase shifter may consist of an input port, followed by a matching section, a variable transmission line section, a matching section and finally an output port. From a manufacturing point of view, some of the best variable transmission line topologies to use at these high frequencies include slot-type transmission lines, such as slotlines and co-planar waveguides (CPW). A slotline can be packaged into a rectangular or circular waveguide, where it is known as a finline, since the conductors around the slots are fin-like protrusions from the waveguide walls. Since a CPW line is essentially just two coupled, parallel slots, all of its properties may also be explained in terms of a single slot or slotline. Further, a CPW line may be more suitable for surface mount packaging.
The tunable material may be tuned by biasing it with a DC voltage across the slot gap. The wider the gap, the higher the biasing voltage needs to be. From a bias voltage control point of view, it is desirable to have a low bias voltage, i.e. a narrow gap. But a narrow gap has a low characteristic transmission line impedance, and is associated with high conductor currents and hence high loss.
Thus, there is a strong need in the phase shifter art for a solution to these conflicting requirements and for improved practical tunable transmission lines.